MEDIUM-POWER PHOTO-MICROGEAPHY 79 



(which are those used in ordinary photography) will be outstanding at another point 

 on the axis. In ordinary photographic lenses this trouble is got over by uniting with 

 the yellow some portion of the violet end of the spectrum, so that when we focus in 

 the yellow we unconsciously focus in the violet at the same time. By this means it is 

 very evident, if the optician has done his work well, our visual focussing will produce 

 a sharp picture on the negative, although taken in reality by the violet ray. There 

 is only one practical way of getting over this difficulty — using visually corrected 

 objectives for photo-micrography — and that is to photograph in monochromatic light 

 and to use a correspondingly stained plate. Yellow light is by far the best, and it works 

 well with the Edward's isochromatic plate ; but what we think much better is to do 

 away with the visually corrected achromatic altogether, and to alone employ objectives 

 corrected for photography. Wray sells an excellent series, and we have often 

 obtained with them the very best results, and with some specimens, when using low- 

 powers, so good as to he almost indistinguishable from those obtained by the use of 

 apochromatics ; although this is not usual unless the object photographed be one with 

 coarse detail, and so not require highN.A. to resolve it.* Those, therefore, who must 

 content themselves with the cheaper form of lenses, are strongly advised to buy achro- 

 matics especially arranged for photography, although they are of very limited use for 

 serious visual purposes. The reader may think this not a little curious, for what can be 

 brighter and more crisp than the image produced by a good photographic lens ? But 

 the fact is, the corrections that have to be applied to a microscopical objective are of 

 a far higher order than those which are sufficient for even the best of photographic 

 lenses, errors left outstanding as negligible in the latter are of quite sufficient import- 

 ance to destroy the best performance of the former. We must not omit to caution 

 those who are commencing the subject not to be led away by purchasing achromatics 

 said to be made for the twofold purpose, visual and photographic. There are optical 

 reasons which must, both theoretically and practically, in the existing state of our 

 knowledge we fear, forbid anything like perfection in these lenses for double use. At 

 their best they are but a compromise, and so do not give the best of results for either 

 purposes. To resume, however, if suitable screens be found and photo-achromatics of 

 the highest order be employed, there are certain specimens such as tissues with coarse 

 markings, that can be worthily shown by photo-micrography. We show one. Fig. i, 

 Plate III., that was taken with a photo-inch by Wray, and the reader must judge 

 for himself as to the quality of the image. 



* This terra N.A. will be explained a little later on, see page 85. 



